Several methods are know for molding footwear to the shape of the foot of the wearer. U.S. Pat. Nos. 3,848,287 and 3,848,286 to Simonsen and Kahmann, respectively, disclose typical processes for adapting a downhill ski boot to conform to the shape of the wearer's foot. In Simonsen and Kahmann, the wearer puts his or her foot in a ski boot formed primarily of thermoplastic material. The ski boot is then placed in a bag of plastic material having a melting point higher than the temperature at which the thermoplastic material of the boot was molded and the top of the bag sealed around the user's foot. After the thermoplastic material of the ski boot is heated to its softening temperature, but below the melting point of the bag, a vacuum applied to a vacuum line inside the bag evacuates air from inside the bag to allow atmospheric pressure to exert a force on the bag and to press the ski boot against the wearer's foot. While the vacuum is maintained inside the bag, the ski boot is cooled to below its softening temperature to produce a ski boot which conforms to the foot of the wearer.
Other methods of manufacturing protective footwear are disclosed in U.S. Pat. Nos. 3,896,202; 4,255,202; 4,724,627; and 4,654,986 to Palau; Swan, Jr.; Sisco; and George, respectively.
Most multi-layer footwear on the market today is manufactured according to a traditional process for making shoes, i.e., thermosetting certain parts of the skate and then manually gluing the component parts together.
A disadvantage inherent in the patents to Simonsen and Kahmann is that their application is limited to ski or other boots made entirely of molded thermoplastic material. If the processes disclosed in these references were used in conjunction with traditionally made multi-layer footwear, according to the process used for manufacturing shoes, the component layers would separate due to deterioration of the adhesives used to secure the laminated layers together.